The present invention relates to a method of forming a tissue paper web in a twin wire former having a rotatable forming roll, said method including the steps of: injecting a jet consisting essentially of an aqueous slurry of papermaking fibers into a converging forming throat formed between two looped forming fabrics as they first converge to meet on a periphery of the rotatable forming roll and then partially wrap the forming roll periphery; sandwiching the aqueous slurry between the two forming fabrics and draining water from the slurry through at least one of them as they partially wrap the forming roll periphery; continuing the draining, and draining a sufficient proportion of water from the slurry to cause the papermaking fibers to form a fibrous web; running the two forming fabrics with the papermaking fibers sandwiched between them up to and around a section of a second roll; and separating one of the two forming fabrics from the formed fibrous web and the other forming fabric no earlier than on said second roll.
In this context, the term "tissue paper" is intended to include any grade of "soft crepe paper" or other paper for sanitary purposes, whether creped or not when used by a consumer.
Such a method is disclosed in U.S. Pat. No. 4,100,018 (Wahren et al.), for example, and is the method inherently used for forming a tissue paper web in a PERIFORMER.RTM.-LW forming section of S-wrap configuration. Due to the rapid drainage of water from the slurry in high speed twin wire formers for tissue making (about 1800 to 2000 meters per minute or higher with a PERIFORMER.RTM.-LW forming section of C-wrap configuration), there is almost no time at all for the papermaking fibers to rearrange themselves or for the paper producer to exert an influence on the formation of the web, that is the manner in which the fibers are distributed, arranged and mixed in the structure of the web. Thus, the formation of a web produced in a twin wire former could be characterized as "frozen" and reflects the relative positions of the fibers in the stock just before the drainage starts. Any disturbance or imperfection at this point will inevitably be found in the web formed on the draining of the slurry. An excellent distribution of the fibers in the slurry will result in an excellent formation, while a less perfect fiber distribution also will result in a less perfect formation and may appear as pin holes or streaking, for example.
In twin wire formers for the production of newsprint and other printing paper grades, such as SPEED-FORMER HS by Valmet Paper Machinery Inc., and the one for the production of lightweight coated grades of paper (LWC) disclosed in U.S. Pat. No. 4,790,909 (Harwood), the speeds used are much lower, about 1300 to 1500 meters per minute and about 900 to 1050 meters per minute, respectively. In these cases the web produced should have as close to one-sided surface properties as possible, that is a minimum of two-sidedness, and the retention of fines and fillers in the surfaces of the web should be comparable to that obtained in a fourdrinier former.
PULP AND PAPER, December 1982, J. C. W. EVANS "New twin wire former designed for maximum fines, solids retention" page 58 discloses a modified new design of the Bel Baie II twin wire former. The new former, called Bel Baie III, is reported to be designed to retain the formation of the Bel Baie II and offer improved solids retention. It is also reported that the Bel Baie II design, which incidentally is disclosed in more detail in United Kingdom Patent No. 1,420,219, for example, is still recommended for all papermaking operations other than those using a high-fines furnish, or for tissue-making where the twin wire tissue former is preferred.